Some electronic applications entail the use of current rectifiers whose optimum voltage properties cannot readily be achieved with an integral number of standard semiconductor rectifying elements. Circuits that simulate PN diodes but have shifted forward voltage characteristics are often employed in these applications.
In discussing such circuits here, the term "V.sub.BE " refers to the voltage across the base-emitter junction of a bipolar transistor. When preceded by "standard", this term means the base-to-emitter voltage of an NPN transistor when it just becomes forwardly conductive. Accordingly, the forward voltage across a conductive PN diode is about 1 standard V.sub.BE.
In a typical case, the threshold voltage of a rectifier must exceed a standard V.sub.BE but must not equal an exact number of (two or more) standard V.sub.BE 's. This case is often handled with a V.sub.BE multiplier in which the base of an NPN transistor is coupled to its collector through one resistor and to its emitter through another resistor. As described in P. Gray et al, Analysis and Design of Analog Integrated Circuits (John Wiley and Sons: 1977), pp. 252-254, the V.sub.BE multiplier acts like a semiconductor diode whose forward conductive voltage equals a standard V.sub.BE multiplied by one plus the ratio of the resistor values.
The situation becomes more difficult if the optimum threshold voltage is less than a standard V.sub.BE. One solution to this problem is described in J. Gunn, "New Techniques In Power Control," 1970 IEEE ISSCC Dig. Tech. Paps., Feb. 19, 1970, pp. 90-91. In Gunn, a simulated diode contains a PNP transistor connected to a transformer in such a manner that appropriate selection of the transformer turns ratio enables the threshold voltage of the diode to drop below a standard V.sub.BE.
Gunn's simulated diode is, however, limited to AC operation. In addition, the transformer is not an element normally made in manufacturing integrated circuits. Modifying the manufacturing process to incorporate the transformer is difficult and costly. Providing the transformer as an element separate from an integrated circuit is unattractive. A circuit capable of acting as a rectifier with a forward voltage of less than a standard V.sub.BE for both AC and DC situations yet manufacturable with standard semiconductor elements is quite desirable.